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Full text of "Text Book Of Mechanical Engineering"


Appendix VL

1127

therefore passing from B to A, then dividing along the arms A H K
and A D K, finally returning by K B. By Ohm's law, E = C R;
and when E is constant C R is constant also. This is the case at
A, for the current is undivided, and must also be the case at H and
D, though with a lower value of E, for none of the current is lost
Therefore

C*  ~R   = C*   T5

Dividing the first equation by the second,

The practical use of the network is to measure one unknown
resistance when the others are known, and in this form becomes
the Wheatstone Bridge, Fig. 997, where the same letters haye
been retained to facilitate understanding. Upon a wood base is
fixed a metre scale I'm, some brass plates ctA.n,j>g, ric£, and a
c slide-wire' ak. A standard known resistance is placed'at R^
joining qr, and the unknown resistance EL is between n and /.
The button D is then moved along the wire till, when depressed,
there is no movement of the galvanometer. Then the resistances
R3 and E.4 are represented by the lengths of metre scale inter-
cepted, and the unknown resistance is

P. = *V

The Thermometer. Fig. 998 shews the present form of resist-
ance thernxometer. A long loop of fine platinum wire K is coiled
round a. piece of mica of cross-shaped section, and the two ends of
the loop are welded to the thicker platinum leads E emerging at
the terminals c D. Ttie object being to measure the Variation in
resistance of the coil K only, a separate loop E, equal in every way
to the leads B, is so connected through F and G to the measuring
apparatus that the final reading is the difference of E'S resistance
from that of B and ic together; and the net result is of course the
resistance of coil K alone. Fig, 999 is the measuring bridge.
The thermometer coil and temperature leads ££, together with the
wire from A. to D, represent the resistance RL: the compensating
leads ee} the wire ED, and adjusting coil r, constitute R2;